Rf/dc generator for quadrupole mass analyzer

ABSTRACT

The specification discloses an improved radio frequency-direct current (RF/DC) generating circuit for use with a quadrupole mass analyzer. A free-running oscillator is employed to generate the radio frequency signal, the oscillator having diode means in the feedback circuit for limiting the dynamic range over which the oscillator must operate. The output of the oscillator is connected through a coil to the rods of the quadrupole mass analyzer which, in conjunction with the coil function as a tuned circuit, thereby the magnitude of the radio frequency applied to the rods is obtained by Q multiplication. A detector circuit is connected to the output of the coil for producing a feedback signal which is compared with a ramp voltage for driving the output of the oscillator as a function of the control ramp input. The detector output is also fed back to the input of two similar circuits for generating equal magnitude with opposite polarity direct current potentials which magnitude varies as a function of the output of the detector and thereby under control of the input control ramp. Additional diodes are provided in the detector circuit to enable the detector to operate near zero volts.

llite ttes atent [1 Lowe [54] lRF/DQ GENEATOR F0 QUADRUPOLE MASS ANALYZER [75] Inventor: Richard Douglas Lowe,.Sunnyvale,

[21] Appl.No.: 139,414

[52] US. Cl ..33l/106, 250/4l.9 DS, 321/18, 331/109, 331/114, 331/117 R, 331/183 [51] lint. Cl. ..H03Ib 3/02 [58] Field of Search ..33l/106, 109, 114, 331/183, 117 R; 250/4l.9 DS; 321/18 [56] References Cited UNYTED STATES PATENTS 3 ,495 ,1 86 3,413,463 ll/l968 Brubaker ..250/41.9 DS

Primary Examiner-Robert Segal Assistant Examiner-Siegfried H. Grimm Attorney-Edward A. Petko and Robert M. Skolnik 2 1970 Wright ..331/109 [451 May 22,1973

[5 7] 7 ABSTRACT The specification discloses an improved radio frequency-direct current (RF/DC) generating circuit for use with a quadrupole mass analyzer. A free-running oscillator is employed to generate the radio frequency signal, the oscillator having diode means in the feedback circuit for limiting the dynamic range over which the oscillator must operate. The output of the oscillator is connected through a coil to the rods of the quadrupole mass analyzer which, in conjunction with the coil function as a tuned circuit, thereby the magnitude of the radio frequency applied to the rods is obtained by Q multiplication. A detector circuit is connected to the output of the coil for producing a feedback signal which is compared with a ramp voltage for driving the output of the oscillator as a function of the control ramp input. The detector output is also fed back to the input of two similar circuits for generating equal magnitude with opposite polarity direct current potentials which magnitude varies as a function of the output of the detector and thereby under control of the input control ramp. Additional diodes are provided in the detector circuit to enable the detector to operate near zero volts.

3 Claims, 2 Drawing Figures ROD SETZ Patented May 22, 1973' 2 Sheet-Sheet 2 N 56 AB 2 2. a J .53 33 60 none Till. .35 33 v Gm mom 2.8: a 6

El /DC GENERATOR FOR QUADRUPOLIE MASS ANALYZER This invention relates to a radio frequency/direct current (RF/DC) generator for use with a quadrupole mass analyzer and particularly to an RF/DC generator having improved characteristics over the circuit disclosed and claimed in commonly assigned copending US. Pat. application, Ser. No. 878,315 filed Nov. 20, 1969, in the names of Bryndza and Wiersma now US. Pat. 3,621,464.

Quadrupole mass analyzers have been described in the prior art, particularly in US. Pat. No. 2,939,952 to Wolfgang Paul et al. In that patent, a controlled varying voltage is used in conjunction with a fixed frequency signal to perform a mass spectrum analysis. Such analyzers may be used in measuring the composition of chemical substances and are primarily comprised of an ionizer, a quadrupole section, and an ion detector. Generally, the substance to be analyzed is introduced into the ionizer as a vapor at low pressure. A portion of the atoms or molecules which make up the chemical substance are ionized by electron bombardment or other means, and these ions are then accelerated and focused into the quadrupole section as an ion beam. The quadrupole beam is filtered by permitting only those ions having specific values of charge-to-mass ratio to pass through the quadrupole section. Those ions which are able to pass through the quadrupole section are then collected by the ion detector which may be an electron multiplier or a Faraday Cup.

The output current produced by the ion detector is a measure of the number of atoms or molecules in the ion beam which have a particular charge-to-mass ratio. The specific detected charge-to-mass ratio is determined by the values of the scanned or controlled varying RF voltage and the related DC voltage which voltages are applied to the electrodes of the quadrupole section. The RF and DC voltages applied to the rods at any given time determine the mass number of the ions being passed by the filter and the ion detector current indicates the ion abundance at that mass number. A display of the quantitative abundance of such detected ions as a function of atomic mass may be conveniently presented on conventional display instruments such as Oscilloscopes, X-Y recorders, or strip chart recorders.

it will be appreciated that precise voltage and frequency control of the radio frequency applied to the mass analyzer is required to obtain maximum resolution. The present invention provides an electronic circuit for providing a fixed frequency ramp of radio frequency voltage and direct voltage for the filter of a quadrupole mass analyzer.

The operation of a quadrupole mass filter requires'4 voltages: 2 voltages are RF which differ in phase by 180 and whose amplitudes are proportional to the mass number being filtered; while the remaining 2 voltages are DC, one positive and one negative. These voltages are generated in the RF/DC generator. A control unit supplies the RF/DC generator with the necessary power, controls the mass number being observed by a 040- volt signal, and switches the generator from normal operation to total pressure by means of a ilS volt signal. The RF/DC generator consists of three loops, one for each of the DC voltages and the third to control an RF oscillator The RF loop consists of a freerunning RF osciallator whose amplitude is voltage limited in proportion to the control signal and which is then stepped up by Q multiplication to the much higher voltage required by the quadrupole mass filter. To stabilize the RF amplitude, a portion of the power fed to the filter is detected and fed back, closing the loop around the oscillator. A portion of this same detected RF signal is also used to control both of the DC loops, thus slaving them to the RF amplitude.

Since at the present state of the art and for practical reasons it is impossible to build a transistor oscillator with an amplitude to 2,400 volts peak to peak, it is necessary to step up the oscillator voltage to that required by the quadrupole mass filter. This can be done most efficiently by using Q multiplication. Since the quadrupole mass filter is a capacitive reactance and dissipates little power because of its high Q, an inductor of the proper value connected in series can be used to series resonate the capacitive load. At resonance the circuit appears as a resistive load where the current is equal to the applied voltage times Q divided by the capacitive reactance, thus the voltage across the filter is Q times the oscillator voltage. The quadrupole mass filter requires that each set of two rods have a signal which is out of phase with respect to the other pair of rods. This is obtained by forming a loop with a single ground.

Two approaches exist for the oscillator. One approach would incorporate a stable oscillator whose output is amplified and then 0 multiplied. Amplitude control would be achieved by controlling the gain of the amplifier. If, however, a slight detuning of the oscillator and Q multiplier circuits were to exist, the Q multiplier would no longer look like just a resisitve load, substantially reducing the current and thus a large loss in output would occur. For this reason, the above approach would be very difficult to align and design stable enough to maintain alignment, or would require a complex frequency compensating scheme.

The approach used in the present invention uses the Q multiplication circuit as the frequency determining element for the oscillator. This approach yields a much simpler circiuit (this is the only tuned circuit in the RF/DC generator); and the resulting instrument is much easier to build and align. The major disadvantage is the resulting requirement of amplitude-controlling the oscillator. Most oscillators can only operate over a very limited dynamic range, yet the mass filter requires a dynamic range of 6 to 2,400 volts peak to peak. The oscillator is a push-pull configuration using very high frequency transistors. Using high frequency transistors allows driving the Q multiplication circuit, or tank with a near square wave (requiring a high beta at high harmonics of the oscillator frequency), thus reducing the dissipation in the transistors.

The circuit includes positive and negative polarity DC generators and a free-running RF oscillator. The direct current is coupled from the respective DC generators directly to the input of a Q multiplication circuit and thence to the quadrupole filter rods. The output of the Q multiplication circuit is detected and fed back to the input of an RF control circuit and also controls the DC loop amplitude. The RF control circuit compares the detected output with a control ramp voltage to sweep the RF output amplitude as a function of the ramp.

The present invention provides novel features for improving the RF/DC generator performance.

These as well as further objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description reference being made to the accompanying drawing in which: the FIGS. 1A and 1B taken together constitute a schematic diagram of the preferred embodiment of the invention.

The oscillator is coupled to the Q multiplication, or tank, circuit L001, through the transformer T202, simplifying the coil design and shielding and permitting easy adjustment of loading and feedback ratios. The Q multiplication circuit is shown in the drawing by inductors L001 and capacitors C1, C2 where the capacitors represent the capacitance of the respective pairs of quadrupole filter rods. The transformer center tap is a RF ground point, simplifying RF bypassing. Feedback to sustain oscillation is derived by series-connecting transformer T201 to the tank. This arrangement allows feedback and thus oscillation only at the resonant frequency of the tank. Resistors R203 and R205 are used to swamp out the input impedance of transistors 0002 and Q003 and limit the base drive for maximum efficiency. Resistor R202, capacitor C203, and resistor R201 are only used for starting; and once the oscillaotr is operating at moderate levels, they may be removed. They initially bias the transistors on, such that the oscillator loop gain is high enough for oscillation. After starting, the transistors change over to class C operation, except at very low levels. Since the most difficult starting condition is at low levels, resistor R201 dominates supplying a constant current. Where the bias from resistor R202 and capacitor C203 is a function of drive level it was found that the combination gave better results than either alone (giving maximum dynamic range and best control). Diode CR201 provides a return for base current.

A circuit arrangement is employed to reduce the range over which the oscillator must work. A pair of back-to-back diodes, CR202 and CR203, with a series resistor, R206, is connected across the feedback winding, reducing the feedback at higher levels. At low levels the diodes are off and the feedback winding is chosen to provide adequate feedback for easy starting and operation at low levels. Since more power is fed back this loads the Q of the circuit, further reducing the voltage to the quadrupole mass filter rods. At medium and high levels the diodes turn on, reducing the feedback and loading on the tuned circuit, thus both optimizing the feedback and switching the gain of the Q multiplier circuit reducing the range over which the oscillator must operate. The turn-off time of the diodes CR202 and CR203 is chosen to be longer than the period, and the diodes once turned on stay on over most of the cycle.

Another significant portion of the circuit is the detector. Since the only knowledge the three loops have of the RF amplitude is through the detector, any nonlinearities or drift in the detector would directly affect the stability of the quadrupole mass analyzer. For example, if the detector efficiency were to change a few tenths of a percent with level or temperature, the RF loop would think that the RF voltage was incorrect and attempt to compensate, producing a shift of the display or mass number being observed by the quadrupole. More importantly, the DC loops would apply the wrong DC levels and a change of only a few tenths of a volt in the DC amplitude could change the peak height read by the quadrupole by orders of magnitude. The RF and DC voltages are combined prior to applying them to the filter. Since it is practically impossible to find fast diodes with a high breakdown voltage and to block this DC voltage from entering the detector, a pair of capacitive dividers with a ratio of 15 to 1 reduces the RF voltage fed tthe detector. This allows the use of diodes with a 200 volt breakdown and 50 nanosecond turn-off time.

The capacitive dividers consist of capacitors C301-C302 and capacitors C303-C304. The voltages at the tap points are clamped to ground by means of diodes CR302 and CR304, whose common point C306 is an RF ground. Thus, for 2,400 volts peak to peak input there exists a signal of volts peak to peak sine wave amplitude clamped to ground at the capacitive divider points. A second pair of diodes CR301 and CR303 peak detect this voltage, forming two halfwave voltage doubler circuits. Since the signals are out of phase by the ripple of the sum signal also adds out of phase, yielding a ripple frequency of twice the RF frequency: thus, the total detector operates as a full wave voltage doubler. Capacitor C305 filters the ripple frequency. Capacitor C307 loads one pair set of quadrupole rods with 5 pf and an equivalent capacitor C001 is across the other pair. C001, being variable, is used to compensate for imbalances in capacitive loading.

In the aforementioned copending application of Bryndza et al., only a pair of diodes such as CR302 and CR304 are used to clamp the RF signal to ground. Thus, in addition to the DC voltage, the full RF voltage is present in the output. This RF voltage must then be filtered, heavily loading the detector output and also providing very severe filtering requirements. In the present invention, the ripple is only a few tenths of a volt.

Because diodes are non-linear, this circuit would normally yield a non-linear output voltage with respect to the input signal. With RF voltages at the rods of 22.5 volts peak to peak or less, which would give l.5 volts peak to peak at the capacitive dividers, or approximately two diode drops, there would be insufficient voltage to turn on the detectors. Thus, there would be no detector output for inputs of 22.5 volts or less, and the detector would be unable to operate at low mass numbers and would have a constant offset throughout the range (thus the desirability of using high voltage diodes). To overcome this difficulty, two additional diodes are added, CR305 and CR306. These diodes are biased on by a current through the resistor R301, yielding a voltage drop of about-1.5 volts at the RF common point. Since CR302 and CR304 are referenced to this point and not ground, all the diodes will be biased on and will start to detect for RF signals near zero volts, thus eliminating the knee effect. Note also that this voltage will track the diode drops with temperature. Any attempts to bias the regular diodes on by simply running current through them will only act as though a signal were present, thus being completely ineffective.

This detected output signal is fed back and compared with the input ramp driving signal to amplifier A102 via resistor R121, thus closing the loop. Resistors R112, R113, and R114 set the loop gain and thus the output of RF voltage for a given input control voltage, R121 protects A102 and compensates for bias current offsets in.A102, and capacitor C110 filters out noise signals which may be present on the control ramp. In addition, it prevents RF signals from following this line back to the control unit. The potnetiometer R110 and resistive dividers R111 and R114 apply an offset voltage to the operational amplifier. Due to the severe problem of starting the RF oscillator and time delay in switching it on, the oscillator is left running during retrace at a minimum level. This minimum level must minimum level. just high enough to sustain oscillation, yet dictates the lower limit to which the unit may respond, thus the need for the start point potentiometer. In addi tion, it compensates for imperfections in the input ramp, i.e., a ramp which does not start at exactly zero volts. Note that if minus l0 volts corresponds to mass 300, mass 1 would be only 33 millivolts.

Transistor Q103 operates in the common base mode to translate the output voltage to the 0-28 volts range required by the emitter-followers and oscillator. Resistors R117 and R115 set the emitter and thus collector current; a change of one volt in the output of A102 would cause a 1 mill change in the collector current, which corresponds to a 3 volt change across R116. CR106 protects the emittenbase junction of 0103 against large reverse biases. The combination transistors 0104 and 0001 act as a Darlington configuration for higher beta or current gain, with the combination acting as an emitter-follower driving the RF oscillator, thus setting the voltage limit point. Since emitterfollowers can be unstable as is Q001 for certain loads, resistor R001 stabilizes the emitter-follower. In addition, it provides a convenient point to monitor the oscillator current, while capacitor C202 bypasses the RF Note that capacitor C202, like the bypassing in the detector circuit, must bypass the RF signal, while not being dominant poles in the loop. Loop compensation being included with the amplifier A102. Resistor R122 helps pull the emitter-follower down to ground and to compensate for leakage current in transistor 0104 at high temperatures.

The DC loops are very similar to each other and the RF loop. For example, amplifier A101 supplies the gain, then transistor Q102 operates as a common base stage, as in the RF loop, translating the output voltage and providing some gain, determined by the ratio of resistors R109 to R108. Diode CR105 protects the emitter-base junction oftransistor 0102. Diode CR103 besides providing protection for the emitter-base junction of transistor 0101 allowsthe loop to pull the output voltage of transistor it is to be noted that the emitterfollower 0101 can only increase the output voltage; it loses control in the opposite direction. The output voltage is then applied to the rods via resistor R207. Capacitor C205 acts as a blocking capacitor to isolate the two DC voltages. The capacitor C109 provides filtering with R207 to prevent the RF from getting back on the emitter-follower. Since the DC was supplied at the low signal point on the inductor L001, the maximum RF amplitude at that point is only about lSO volts, primarily due to the voltage increase across capacitor C205, (the blocking capacitor). If C205 were too small, the voltage drop across it would be very large, reducing the amount of voltage gain available across L001 and increasing the filtering problems. If C205 were too large, it would set the maximum rate of change of the DC signal and thus limit the response time of the RF/DC generator. Again transistor @101 is unstable for large capacitive loads, thus capacitor C109 must be realtively small or the emitter-follower will oscillate.

The feedback around the loop consists of resistor R120 and the voltage divider resistors R124, R129, and

. Since the required DC to RF ratio is not quite a constant over the total mass range, diode CR101 turns on at approximately mass 10, reducing the RF/DC ratio, or the DC loop gains by the ratio of R101 shunted across resistors R102 and R103. An additional slope change is provided by the resistive divider R105, the slope adjustment R104 and the diode CR102. This can be user set in the high mass range, and can also be used to compensate for a loss in sensitivity of the mass filter at high mass numbers. Resistor R106 reduces the effect of offset and bias changes with temperature in operational amplifier A101. The negative DC loop has in addition an offset control resistor R126 with divider resistors R127, R128, and gain resistor R131. This allows for compensation of differences in input offset voltage between the two operational amplifiers A101 and A103, in other Words, tracking of the DC voltages at low massnumbers. The absolute compensation is nulled by the ratio adjustment which also changes the RF/DC ratio by an offset. This voltage ranges from l0 to +10 volts and is summed with the DC ramps through R118 and R136.

The ratio control sets the DC to RF ratio or slope. To adjust for differences in gain or slope in the DC loops, due to component tolerances, a balance potentiometer R129 is provided. The gain of the positive loop, for example, is determined by equating the current in resistor R107 to that in resistor R119. The total current through resistor R120 equals the DC rod voltage divided by resistor R120 plus the parallel combination of resistor R119 and the shunt branch resistor R124 plus resistor R129. Thus, if resisotr R129 were adjusted so as to decrease the resistance in the positive loop, this would shunt more current and thus increase the gain in the plus DC loop. However, decreasing the resistance in the positive DC loop would increase that in. the negative loop, thus changing the gain in a differential fashion. This control has the greatest effect at high mass numbers, while the offset balances the loops at low mass numbers.

The DC loops are switched off by diodes CR104 and CR107, resistor R123, and capcitor C107 yielding zero DC rod voltages for total pressure operation. In normal operation CR104 and CR107 are biased off, CR107 by the application of 15 volts by switch and diode CR104 by th detected RF voltage. In the total pressure position +15 volts is applied by the switch, turning both CR104 and CR107 on. This applies a plus 0.6 volts to the positive and negative rod driver amplifiers. Since the DC loops cannot respond to a positive input, the DC rod voltages stay at zero and the opeational amplifiers latch up. R123 sets the diode on current and C107 bypasses noise and prevents RF signals from following this line back to the control unit.

For generation of the DC voltage, a negative DC loop is provided which is similar in configuration to the positive DC loop discussed above. In the DC loop, amplifier A103 supplies gain for the common base transistor (2106 via resistor R134 which acts as a voltage translator. Diodes CR109 and CR108 correspond in function to previously discussed diodes CR and CR103, respectively. The output voltage DC is applied to the quadrupole rods via a resistor, R204. The capacitor C113 provides filtering with resistor R204 preventing the RF from backing on the emitterfollower stage.

The feedback path around the loop for the -DC generator includes a resistor R133 and voltage dividing resistors, R130, R132, and R129. These resistors with the resistors R135, R137, R102 and R103 control the set of the RF to DC ratio. The transistor Q105 and the resistor R125 correspond to transistor 0101 and resistor R109, respectively. Capacitors C103 and C112 serves as 250 volt bypass capacitors.

Capacitor C117 serves as to bypass the resolution control voltage to eliminate noise. Other bypass capacitors C201, C204 and C106 are connected to the +28 volt sources to bypass this supply and to provide a low impedance path to ground for any RF on the line.

1 claim:

1. An improved radio frequency-direct current generating circuit comprising:

output means having frequency selective characteristics;

free-running oscillator means connected to said output means for generating an output signal at a frequency determined by said output means, said oscillator means including first and second transistors connected in push-pull relationship, feedback means connected to said transistors for developing a first control signal, and diode means connected in said feedback means for reducing the magnitude of said first control signal thereby controlling the range of operation of said oscillator;

detector means coupled to said oscillator means for producing a second control signal, said detector including first and second paired diodes for rectifying said oscillator outputs, and further diode means connected between said detector and ground for enabling said detector to operate at close to zero volts;

means for producing a ramp voltage;

comparison means connected to said ramp producing means and to said detector means for producing an output signal having a magnitude representing the difference in magnitude between said ramp and said second control signal;

means connected between said comparison means and said oscillator to control the output of said oscillator as a function of said output signal; first and second direct current generating means for generating opposite polarity direct current potentials, each of said direct current generating means including potentiometer means for generating a direct current control signal, and second comparison means connected to said potentiometer means and to said detector means for producing a direct current output having a magnitude which varies as a function of said direct current control signal; and

means connected to said second comparison means for applying said direct current outputs to said output means.

2. An electrical circuit for generating an output signal having radio frequency and direct current components said circuit comprising:

free-running oscillator means for generating a radio frequency electrical signal;

output means having frequency selective properties coupled to said oscillator means for receiving said radio frequency signal and for establishing the amplitude of said radio frequency signal by Q- multiplication;

detector means connected between said oscillator means and said output means for producing a feedback signal having an amplitude proportional to the amplitude of said radio frequency signal, said detector means including means to enable said detector to operate at close to zero volts;

a variable amplitude control signal source;

comparison means connected to receive said variable amplitude control signal and said feedback signal for producing an output indication proportional to the difference in amplitude therebetween;

means connected to said comparison means and to said oscillator means for controlling the amplitude of said radio frequency signal as a function of said output indication;

and means connected to said output means for producing opposite polarity controlled amplitude direct current signals for application to said output means;

said free-running oscillator means including resonant circuit means including said output means and inductor means coupled thereto, complementary connected transistor means operating in push-pull mode connected to said inductor means, means connected to said resonant circuit and to said transistor means for producing a driving signal for said transistors, and diode means connected in said last mentioned means for enabling oscillation at low amplitude driving signals.

3. An improved radio frequency/direct current generating circuit comprising:

frequency selective output means for receiving a radio frequency/direct current signal;

radio frequency signal generating means for producing a radio frequency signal by Q-multiplication including said frequency selective output means active means adtive means for driving said selective means;

means for producing a variable control voltage;

means connected to said control voltage producing means and to said output means for producing a difference signal representing the difference in amplitude between said radio frequency signal and said control voltage;

means responsive to said difference signal for controlling the amplitude of said radio frequency signal as a function of said control voltage; and

direct current signal producing means connected to said output means for supplying direct current signals thereto said direct current producing means including potentiometer means for generating a direct current control signal and comparison means connected to said potentiometer means and to said frequency selective output means for producing a direct current output which varies as a function of said direct current control signal.

t it t 

1. An improved radio frequency-direct current generating circuit comprising: output means having frequency selective characteristics; free-running oscillator means connected to said output means for generating an output signal at a frequency determined by said output means, said oscillator means including first and second transistors connected in push-pull relationship, feedback means connected to said transistors for developing a first control signal, and diode means connected in said feedback means for reducing the magnitude of said first control signal thereby controlling the range of operation of said oscillator; detector means coupled to said oscillator means for producing a second control signal, said detector including first and second paired diodes for rectifying said oscillator outputs, and further diode means connected between said detector and ground for enabling said detector to operate at close to zero volts; means for producing a ramp voltage; comparison means connected to said ramp producing means and to said detector means for producing an output signal having a magnitude representing the difference in magnitude between said ramp and said second control signal; means connected between said comparison means and said oscillator to control the output of said oscillator as a function of said output signal; first and second direct current generating means for generating opposite polarity direct current potentials, each of said direct current generating means including potentiometer means for generating a direct current control signal, and second comparison means connected to said potentiometer means and to said detector means for producing a direct current output having a magnitude which varies as a function of said direct current control signal; and means connected to said second comparison means for applying said direct current outputs to said output means.
 2. An electrical circuit for generating an output signal having radio frequency and direct current components said circuit comprising: free-running oscillator means for generating a radio frequency electrical signal; output means having frequency selective properties coupled to said oscillator means for receiving said radio frequency signal and for establishing the amplitude of said radio frequency signal by Q-multiplication; detector means connected between said oscillator means and said output means for producing a feedback signal having an amplitude proportional to the amplitude of said radio frequency signal, said detector means including means to enable said detector to operate at close to zero volts; a variable amplitude control signal source; comparison means connected to receive said variable amplitude control signal and said feedback signal for producing an output indication proportional to the difference in amplitude therebetween; means connected to said comparison means and to said oscillator means for controlling the amplitude of said radio frequency signal as a function of said output indication; and means connected to said output means for producing opposite polarity controlled amplitude direct current signals for application to said output means; said free-running oscillator means including resonant circuit means including said output means and inductor means coupled thereto, complementary connected transistor means operating in push-pull mode connected to said inductor means, means connected to said resonant circuit and to saId transistor means for producing a driving signal for said transistors, and diode means connected in said last mentioned means for enabling oscillation at low amplitude driving signals.
 3. An improved radio frequency/direct current generating circuit comprising: frequency selective output means for receiving a radio frequency/direct current signal; radio frequency signal generating means for producing a radio frequency signal by Q-multiplication including said frequency selective output means active means adtive means for driving said selective means; means for producing a variable control voltage; means connected to said control voltage producing means and to said output means for producing a difference signal representing the difference in amplitude between said radio frequency signal and said control voltage; means responsive to said difference signal for controlling the amplitude of said radio frequency signal as a function of said control voltage; and direct current signal producing means connected to said output means for supplying direct current signals thereto said direct current producing means including potentiometer means for generating a direct current control signal and comparison means connected to said potentiometer means and to said frequency selective output means for producing a direct current output which varies as a function of said direct current control signal. 